Supporters of Iran's main pro-reform presidential candidate formed a human chain that stretched nearly the entire length of Teheran in their biggest display of political might - sending a powerful challenge to President Mahmoud Ahmadinejad's backers as both sides poured into the streets in the final days of the race. The showdown atmosphere reflects the increasingly bitter tone between Ahmadinejad and his main rival, Mir Hossein Mousavi, in the campaign blitz before Friday's vote. Both have traded recriminations and engaged in mudslinging as intense as any bare-knuckle American campaign and - in the process - have turned the election into a display of Iran's deep political divides. Ahmadinejad's supporters applaud his firebrand style that include questions over the Holocaust and his uncompromising defense of Iran's nuclear program. Mousavi's backers - including many young voters - believe Iran's international standing is being undermined and a more flexible approach is needed with critical issues at stake such as possible talks with Washington. In the battle for campaign images, Mousavi's forces mobilized a stunning scene Monday: a rally that stretched nearly the entire 12-mile (19-kilometer) length of Vali Asr - a famous avenue that bisects Tehran from the conservative strongholds in the older flatland neighborhoods to the south from the liberal-minded bastions on the slopes of the Alborz mountain range in the north. The road - shaded in many places by towering plane trees - was turned into a river of green by Mousavi supporters carrying banners, head scarfs, ribbons and anything else in the campaign's trademark color. "This is a message to all of Tehran's population," said Sharan Kjarimi, 32, an industrial engineer who joined the rally. One man fashioned a sign copying an Iranian newspaper front page the day the Western-back shah left the country with the 1979 Islamic Revolution poised for victory. But the headline was changed: "Ahmadinejad has left." Others chanted "Ahmadini bye-bye" and "If they don't cheat, Mousavi will win." Mousavi addressed the crowd from southern tail of the rally: "We've gathered here because people are tired of lies ... the human chain is a symbolic rejection of lies being said to the people." But Ahmadinejad's bloc staged its own show of unity, using Iran's flag as their banner and patriotism as their cry. At one point, Ahmadinejad's supporters and Mousavi loyalists faced off with each side shouting slogans and waving their respective flags. Tens of thousands of Ahmadinejad backers gathered at Tehran's largest prayer hall - joining in a mass denunciation of Mousavi, United States and Israel. The government spokesman, Gholam Hossein Elha, claimed Ahmadinejad tried to make his way through the crowd for a speech but it was too vast. Ali Rezae, a supporter of the president, praised him for fighting on behalf of Iran's interests. "He won our national dignity in international arenas," said Rezae. There are no credible political polls in Iran, but both campaigns predict what a very tight race. The head of Iran's election board, Kamran Daneshjoo, also said the voter turnout could surpass the 79.93 percent in 1997 when reformist Mohammad Khatami came to power. The reformists are counting on a large turnout - particularly from young voters - to overcome Ahmadinejad's core support from working-class families and the powerful Revolutionary Guard, which has the power to control millions of votes through its nationwide volunteer corps. The tone of the campaign has become increasingly sharp as each side looks to gain any advantage. On Monday, reformists launched their latest barrage: distributing a video of Ahmadinejad claiming a "light" surrounded him during a U.N. address in 2005. In the clip, sent out e-mail and on CDs, Ahmadinejad tells a top cleric, Ayatollah Abdollah Javadi Amoli, that a "light" enveloped him during his address to the U.N. General Assembly and that the crowd stared without blinking during the entire speech. "A member of the (Iranian) delegation told me, 'I saw a light that surrounded you,'" Ahmadinejad said. "I sensed it myself too ... I felt the atmosphere changed. All leaders in audience didn't blink for 27, 28 minutes. I'm not exaggerating when I'm saying they didn't blink. Everybody had been astonished ... they had opened their eyes and ears to see what is the message from the Islamic Republic." The clip was released after Ahmadinejad on Saturday denied making the comment. Mousavi accused Ahmadinejad of being "superstitious" and "brazenly staring at the camera and telling lies to the nation." On Saturday, Ahmadinejad said inflation stood at 15 percent, but Mousavi showed a report released by the Central Bank of Iran indicating it stood at 25 percent. "Why do we lie to people? Why do we give people wrong information? Is this to the country's benefit? Is gaining the presidential chair worth lying to people this blatantly?" Mousavi said on Sunday. Reformists, who promise to ease social and political restrictions at home and seek better ties with the West, appear to be gaining ground on Ahmadinejad, who has become increasingly unpopular because of Iran's economic woes. Critics also say he has needlessly enflamed world anger at Iran with his statements calling U.N. resolutions "worthless papers" and casting doubt on the Holocaust. There are two other candidates in the race. Former parliament speaker Mahdi Karroubi, who is considered a moderate, could siphon some votes from Mousavi. Mohsen Rezaei, a former commander of the powerful Revolutionary Guard, threatens to undercut Ahmadinejad's conservative base. Ahmadinejad's comments also have become the source of political satire that takes aim at his pious reputation among his supporters. "Have you seen a halo in your addresses?" former vice president, Mohammad Ali Abtahi, asked Karroubi during a documentary shown on state TV last week. "Only certain people can see that. I don't have this spiritual status," Karroubi replied.